Abstract:
Cold spells and heatwaves, like the ones experienced by Europe and North America, have severe ecological, economic and health impacts and are linked to a peculiar phenomenon known as atmosphere blocking. An atmospheric block is an anomalously persistent high-pressure system in the midlatitudes (30-60 deg N/S), resulting in extreme cold days in the winter and warm days in the summer. Despite decades of research there has been no consensus on the mechanisms that lead to atmospheric blocks. A recent theoretical breakthrough by Nakamura and Huang (2018) offered an exciting path forward by drawing an analogy between blocking in the atmosphere and traffic jams on a highway.
In my talk I will show that atmospheric blocks are persistent waves in the midlatitude jet stream that follow conservation properties of ‘finite amplitude local wave activity (LWA)’, a new wave metric that measures the strength of large-amplitude Rossby waves. Using a feature tracking algorithm on ERA5 Reanalysis dataset and the LWA budget, we identify every possible persistent event that has occurred during the Northern Hemisphere winter from 1979-2021. Most of these events are found to occur in clusters collocated with the quasi-stationary ridge over the Euro-Atlantic sector and the Pacific sector. Despite substantial variations among individual Rossby-wave packets, on an average these events exhibit a non-linear relationship between the wave activity and wave activity flux and tend to persist in regions with low flux capacity - a characteristic traffic jam behavior in the atmosphere. Finally using a simplified version of the LWA budget, I will discuss how blocking characteristics can potentially change in response to hypothetical climate change scenarios.